Patent Documents 1 to 3 disclose apparatuses for detecting whether a person is present by intermittently transmitting an ultrasound wave in a monitoring space, receiving a reflected wave of the ultrasound wave, and comparing the waveform of a received signal corresponding to the reflected wave and the waveform of a received signal obtained when no person is present.
FIG. 1 is a block diagram illustrating a configuration of an ultrasonic human body sensor disclosed in Patent Document 1. Referring to FIG. 1, an ultrasonic human body sensor 10 includes an oscillation unit 1, a boosting circuit 2, an ultrasonic wave transmission/receiving unit 3, a receiving/detection circuit 4, a received waveform conversion unit 5, a reference waveform storage unit 6, and a determination unit 7.
The oscillation unit 1 for generating an ultrasonic pulse signal is controlled by the determination unit 7. A generated ultrasonic range pulse is output to the boosting circuit 2. The determination unit 7 controls the oscillation unit 1 so that an ultrasonic range pulse continuing for a predetermined period is intermittently output at predetermined time intervals.
The voltage of the ultrasonic range pulse output from the oscillation unit 1 is raised by the boosting circuit 2. The ultrasonic wave transmission/receiving unit 3 intermittently transmits an ultrasonic wave continuing for a predetermined period in monitoring space with an ultrasonic range pulse signal having a raised high voltage. A received signal corresponding to a reflected wave of each transmitted ultrasonic wave is detected by the receiving/detection circuit 4 and is then input into the received waveform conversion unit 5.
The received signals corresponding to reflected waves of ultrasonic waves that have been intermittently transmitted are subjected to receiving masking, are sequentially sampled at predetermined time intervals, are subjected to A/D conversion by an A/D conversion unit or the like included in the received waveform conversion unit 5, and are converted into received waveform data composed of a plurality of digital sampling values.
The determination unit 7 compares the received waveform data with reference waveform data and outputs a detection signal when detecting that a human body is present in the monitoring space. More specifically, the determination unit 7 calculates the difference between a received waveform stored in the reference waveform storage unit which is obtained when no person is present and a current waveform for each A/D sampling value and calculates the sum of these differences. When the sum of these differences is equal to or larger than a threshold value set in advance, the determination unit 7 determines that a human body is present. At the time of determination, the determination unit 7 divides the received waveform into a near region and a far region and calculates the sum of differences in each of the near region and the far region. As a result, determination is more accurately performed.
FIG. 2 is a waveform diagram illustrating an operation of an ultrasonic sensor disclosed in Patent Document 2. An ultrasonic sensor disclosed in Patent Document 2 outputs “1” when a reflected wave is present in a minute zone and “0” when no reflected wave is present and compares reference data of a reflected wave with a received signal in each minute zone.
In a sensor disclosed in Patent Document 3, a transmitter intermittently transmits an ultrasonic wave in monitoring space, a receiver receives a reflected wave generated by the reflection of the transmitted ultrasonic wave from an object, a binarization circuit converts the waveform of the received reflected wave into current binarized information, a comparison circuit compares the current binarized information with binarized information stored in advance in a storage device which has been obtained when no person is present in the monitoring space, and it is determined that a person is present in the monitoring space when the current binarized information differs from the stored binarized information.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2006-343218    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 5-27028    [Patent Document 3] Japanese Unexamined Patent Application Publication No. 56-160673
However, the sensors disclosed in Patent Documents 1 to 3 need storage devices having a capacity for storing the waveforms of reflected waves obtained in a whole measurement period. Accordingly, in the case of a wide monitoring area or a monitoring area in which a reflected wave stays for a long period, a memory with a large capacity is required. This leads to the increase in cost and size.